1. Field of the Invention
The present invention relates generally to telecommunications, and more specifically, to wireless communications.
2. Background
In an enhanced reverse link scheme, four different types of packets may be transmitted from a mobile station or an access terminal (AT) to a base station. The packets transmitted through the reverse link may have different physical layer packet lengths, for example, four slots, eight slots, twelve slots or sixteen slots, depending on the type of the packet. An access terminal may desire to transmit a packet shorter than sixteen slots while still achieving the same packet error rate (PER) as that of a sixteen-slot packet. For example, it may be desirable to achieve a PER of 1% regardless of the number of slots in the transmitted packet.
When an AT transmits a packet that has a length of less than four subpackets, each of the subpackets consisting of four slots, the AT boosts the traffic-to-pilot (T2P) ratio of the transmitted subpackets to overcome the loss of ratio of bit energy to noise power spectral density (Eb/N0), coding gain and time diversity. The T2P ratio is conventionally defined as the ratio of the chip energy of the data channel (Ec, data) to the chip energy of the pilot (Ecp).
The base station may be able to estimate the physical layer payload size by decoding a reverse rate indicator (RRI) channel in a conventional manner known to a person skilled in the art. However, the base station does not know a priori whether the received subpacket is boosted in power by the AT. The lack of a priori knowledge by the base station may cause a problem in Turbo decoding because unlike a conventional Viterbi convolutional decoder, a Turbo decoder requires knowledge of the signal-to-noise ratio (SNR) information in computing a log-likelihood ratio (LLR). Hence, some mechanism is needed for properly computing the LLR for the Turbo decoder.
The LLR required for Turbo decoding is typically proportional to the SNR of demodulated data received by the base station, which may be expressed as the ratio of chip energy of the data (Ec, data) to the effective noise power spectral density (Nt) at the base station antenna. Because demodulated data obtained by maximal ratio combining (MRC) are proportional to (Ecp/Nt)·sqrt(T2P), the demodulated data symbol needs to be scaled by the square root of T2P in a conventional scheme of LLR computation. A typical approach of obtaining the LLR for the Turbo decoder is to calculate the estimated T2P ratio from the ratio of SNR of the data symbols to that of the pilot symbols every time a subpacket is received by the base station and then to map the estimated T2P ratio to one of the four choices of packet lengths. However, this approach entails setting decision boundaries which may vary according to the type of physical channel and may be computationally inefficient.
Therefore, there is a need in the art for a more efficient but near-optimal approach to LLR computation for the Turbo decoder.